It is known to determine the airspeed V of an aircraft on the basis of pressure measurements of the air around the aircraft. More precisely, by measuring the total pressure Pt for example with the aid of a Pitot tube, and the static pressure Ps of the air, the airspeed V of the aircraft can be determined by the following equation
      Pt    -    Ps    =      ρ    ⁢                  ⁢                  V        2            2      where ρ represents the density of the air.
At low airspeed, this measurement method gives unreliable results. This is because the square of the airspeed is proportional to the pressure difference Pt−Ps. A small error in the pressure measurements consequently leads to a large error in the airspeed. More precisely, differentiating the equation cited above gives:
            d      ⁡              (                  Pt          -          Ps                )              =          ρ      ·      V      ·      dV        or      dv    =                  d        ⁡                  (                      Pt            -            Ps                    )                            ρ        ·        V            
This shows that a finite error d(Pt−Ps) in the pressure measurements leads to an infinite error dV in the airspeed determination when the velocity is zero, or a large error in the airspeed determination when the airspeed is low. For example, at 10 knots of true airspeed, an error of 1 hPa in the pressure measurements leads to an error of 30 knots in the airspeed determination. When the error is larger than the measurement, the result is unacceptable.
Optical measurement devices, or lidars, can also make it possible to determine the velocity and the direction of the airflow by detecting the Doppler effect in a back-scattered signal of a light beam emitted by the measurement device into a medium which is intended to be analyzed. The Doppler effect consists in a frequency shift of a light wave reflected by a moving object. The distance between the measurement device and the object, for example a group of moving particles, defines the type of detection of the Doppler shift, which may be of the coherent type or of the direct or incoherent type depending on the case. These devices rely on the presence of particles in the medium in which the optical measurements are carried out, and they are very difficult to tune.
For helicopters, a propeller rotating above the rotor has been developed. This propeller is fixed on the top of a tube integrally connected to the main gearbox of the helicopter. The propeller, fitted with a Venturi tube at each end, rotates at approximately 720 rpm above the main rotor hub. The velocity of the airflow is determined by the position in amplitude and phase of the pressure variation between each venturi relative to the angular position. The system in question is very cumbersome to install on a helicopter and is not widely used. It has the same defects as the Pitot tube at low airspeeds.
Again for helicopters, a synthetic anemometry concept has been developed which makes it possible to provide more reliable velocity information of the airflow at low airspeeds. This concept is based on the principle that the velocity of the air is proportional to the difference between the cyclic pitch of the rotor lifting the helicopter and the attitude of the helicopter. This principle makes it possible to determine both the direction and the modulus of the velocity vector of the air. This method has only been used to date in flight test centres in order to develop new helicopters. This method has not been employed in normal use of helicopters, essentially because of its high development cost and calibration difficulties.